not wanting to be an armchair general, since i don't understand much about military things.

i make a guess that china has logistical advantage, continuous delivering ammo and weapons, they have a insanely strong industrial capability, and in a time of war that can be converted to, high speed trains make sure the equipment arrives safe and fast, while usa produced equipment would need to be delivered mostly by the pacific ocean since they don't have a reliable way to deliver to a landmass close to taiwan and just finish the deliver by water, and chinas navy is very active in the pacific which can make such deliveries hard, i think for something to be delivered safe and quick, would be by diverting equipment from south korea or japan